Arsenic contamination of drinking water poses a substantial threat to public health (Waldman, 2001). Concentrations of arsenic in groundwater in some areas are elevated as a result of erosion from local rocks. In addition, the combustion of fossil fuels and industrial effluents (arsenic is a by-product of industrial processes including semiconductor manufacturing, petroleum refining, and mining and smelting operations) contribute to arsenic contamination in drinking water. Severe health effects have been observed in populations drinking arsenic-rich water over long periods in many countries. According to the Environmental Protection Agency (EPA), international studies have linked long-term exposure to arsenic in drinking water to cancer of the bladder, lungs, skin, e.g., basal cell carcinoma, squamous cell carcinoma, kidney, nasal passages, liver and prostate (Walsh, 2001). Arsenic exposure has also been linked to numerous other deleterious effects on various organ systems of the human body, e.g., skin (hyperkeratosis of palms and soles, melanosis or depigmentation, Bowen's disease), liver (enlargement of the liver, jaundice, cirrhosis, non-cirrhotic portal hypertension), nervous system (peripheral neuropathy, hearing loss), cardiovascular system (acrocyanosis, Raynaud's phenomenon), the respiratory system as well as the endocrine system (diabetes mellitus and goiter).
On Oct. 31, 2001, the EPA announced its decision to move forward in revising the existing 50 parts per billion (ppb) standard for arsenic in drinking water by implementing a maximum contaminant level (MCL), or regulatory level, of 10 parts per billion (ppb). Water systems will have to meet this new standard by January 2006.
In water, the most common valence states of arsenic are As(V), or arsenate, which is more prevalent in aerobic surface waters and As(III), or arsenite, which is more likely to occur in anaerobic ground waters. In the pH range of 4 to 10, the predominant As (III) compound is As(OH)3, which is neutral in charge, while the As (V) species are negatively charged. Removal efficiencies for As(III) are poor compared to removal As(V) by existing technologies due to the lack of negative charge. As (III) may be converted to As(V) by oxidation. Ferric chloride and potassium permanganate are effective in oxidizing As(III) to As(V). However, pre-oxidation with chlorine may create undesirable concentrations of disinfection by-products.
Coagulation/filtration (C/F), the standard treatment for remediating arsenic and other contaminants from surface water, uses iron that reacts with arsenic salts to create a solid that precipitates from the water. The type of coagulant and dosage used affects the efficiency of the process. Within either high or low pH ranges, the efficiency of C/F is significantly reduced. Moreover, this treatment system requires large settling tanks, and produces an arsenic-contaminated sludge that may need to be disposed of in a hazardous waste landfill based upon the revised EPA arsenic standard. C/F is not appropriate for most small water treatment systems due to high costs associated with the technique, as well as the need for well trained operators. In addition, there is variability in process performance of C/F treatment systems.
Another remediation technique, lime softening (LS), can provide a high percentage of arsenic removal for influent concentrations of 50 μg/L when operated within the optimum pH range of greater than 10.5. However, as with C/F, LS is not applicable for small water treatment systems due to the prohibitive costs associated with the technique. Moreover, it may be difficult to consistently meet a low-level MCL using either C/F or LS alone. Disposal of contaminated sludge generated during L/S may also present a problem
Activated alumina (AA) is effective in treating water with high total dissolved solids (TDS). However, selenium, fluoride, chloride, and sulfate, if present at high levels, may compete for adsorption sites. AA is highly selective towards As(V), and this strong attraction results in regeneration problems, possibly resulting in 5 to 10 percent loss of adsorptive capacity for each run. Drawbacks to this technique also include the lack of availability of F-1 alumina. In addition, chemical handling requirements may make this process too complex and dangerous for many small systems. Moreover, AA may not be efficient in the long term, as it seems to lose significant adsorptive capacity with each regeneration cycle, and disposal of highly concentrated waste streams produced by this technique may also be problematic.
Ion exchange (IE), e.g., anion exchange, technology can effectively remove arsenic from contaminated water. However, sulfate, TDS, selenium, fluoride, and nitrate compete with arsenic for exchange and can affect run length. Suspended solids and precipitated iron can cause clogging of the IE bed. Thus, systems containing high levels of these constituents may require pretreatment. Anion exchange involves passing water with anions of arsenate through a column of resin beads containing exchangeable, innocuous ions such as chloride, resulting in an exchange that leaves the arsenate in the beads and the chloride in the water. IE will not remove uncharged compounds, which means it will not work with uncharged As(III) unless it is pre-oxidized. In addition, removing contaminants at lower levels will affect how soon the exchange column must be regenerated before breakthrough (the point at which removal levels begin to deteriorate). In addition, IE produces an arsenic-contaminated brine, the disposal of which may be problematic.
Another common water treatment approach, reverse osmosis (RO) involves pushing water through a membrane that captures contaminants. Although effective in removing contaminants to below 2 μg/l, RO is a more expensive technology than C/F and produces a brine that itself must be treated for arsenic contamination. Moreover, RO produces a larger waste stream than other treatment methods, which may make the method impractical in locales where water is scarce.
Additional methodologies currently employed for water remediation include electrodialysis reversal (EDR) and nanofiltration (NF). EDR is expected to achieve removal efficiencies of 80 percent. In one study, NF was capable of arsenic removals of over 90%, however, a recent study showed that the removal efficiency dropped significantly during pilot-scale tests where the process was operated at more realistic recoveries. With either of these techniques, water rejection (about 20-25 percent of influent) may be an issue in water-scarce regions. Moreover, EDR may not be competitive with respect to costs and process efficiency when compared with RO and NF.
In bioremediation methodology, microorganisms, both naturally occurring and/or genetically engineered, are used as agents to remove contaminants such as organic compounds, e.g., petroleum hydrocarbons, from soils and water. This technology has recently emerged as a viable remediation method (Ritter and Scarborough, 1995). Additionally, in recent years phytoremediation, the use of plants and trees to clean up contaminated soil and water, has been used to treat hazardous wastes. For example, U.S. Pat. No. 6,166,290 discusses the use of transgenic plants, i.e., plants encoding a glutathione S-conjugate (GS-X) pump, e.g., the yeast vacuolar Ycf1p pump, for the bioremediation of contaminated soils.
As there are several disadvantages associated with the remediation techniques currently employed to remove arsenic from water, there is a need for a bioremediation method that removes arsenic from contaminated water.